The series of process steps used to fabricate semiconductors and other electronic devices commonly include various plasma processes for depositing or etching films. Such plasma processes generally are performed within a vacuum chamber whose interior is maintained at a low pressure by an exhaust pump. The pump is coupled to the chamber interior via an exhaust channel.
There are several reasons that is important to prevent the plasma within the chamber from extending through the exhaust channel to the exhaust pump.
One reason is that a plasma that extends a considerable distance through the exhaust channel typically is unstable, which causes the main plasma body adjacent the workpiece to be unstable, as evidenced by visible flickering of the light emitted by the plasma. Such instability produces unacceptable non-uniformities in the plasma process.
A second reason it is important to block the plasma from extending to the exhaust pump is to protect the pump from corrosion or undesirable deposits. Any component surfaces near the plasma body within a plasma chamber generally will be corroded due to bombardment by ions from the plasma. In addition, many plasma processes have byproducts which include reactive species that can combine to form polymers or other undesirable deposits on components within the chamber that are adjacent to the plasma body. To prevent such corrosion and undesirable deposits, it is common practice to cover with removable liners the surfaces of chamber components that are exposed to or adjacent to the plasma body. The liners are replaced periodically as they become corroded.
Typically it is impractical to provide easily replaceable components for the exhaust pump. Therefore, a need exists for a plasma chamber design that prevents the plasma within the chamber from extending too close to the exhaust pump.
Commonly assigned U.S. Pat. No. 5,891,350, issued Apr. 6, 1999 to Shan et al., discloses a plasma chamber design that prevents the plasma from reaching the exhaust pump by interposing a long and sinuous exhaust passage between the chamber interior and the pump. One disadvantage of this design is that the sinuous exhaust passage creates a pressure drop between the chamber interior and the pump. In some applications, this may require an overly expensive pump to achieve a desired low chamber pressure.